Thanks for the query to H.C.M. Forum.
The purple coloration may be due hamper in blood supply to the part.
2nd reason may be due to nerve
involvement ,so for this confirmation an arteriography is important of parts involved.
Bluish coloration in body itself indicates that there is interfere in the blood supply .
You have not mentioned the age , diabetic history ,hypertension
history so it is very difficult to say why there was obstruction in blood supply.
So consult a physician and get his opinion.